Be sure that the vac modulator valve and the plastic pluger behind it are in good shape and are the correct ones for the application. Mercedes has made many of them over the years that are the same in appearance but different in function.
Also be sure that the vacuum at the valve is good. And the cap has to be on and in new shape.
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM